Scanning electron microscope

ABSTRACT

A scanning microscope is provided for producing a scan image at high spatial resolution and in a low acceleration voltage area. An acceleration tube is located in an electron beam path of an objective lens for applying a post-acceleration voltage of the primary electron beam. The application of an overlapping voltage onto a sample allows a retarding electric field against the primary electron beam to be formed between the acceleration tube and the sample. The secondary electrons generated from the sample and the secondary signals such as reflected electrons are extracted into the acceleration tube through the effect of an electric field (retarding electric field) immediately before the sample. The signals are detected by secondary signal detectors located upwardly than the acceleration tube.

This is a division of U.S. patent application Ser. No. 08/733,857, filed18 Oct. 1996, now U.S. Pat. No. 5,872,358, the entirety of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates in general to a scanning electronmicroscope for obtaining a two-dimensional scanning image showing ashape, a composition or the like of a surface of a sample by scanningthe surface of the sample to be inspected with an electron beam, andmore particularly to a scanning electron microscope which is suitablefor obtaining a scanning image having high resolution in a lowacceleration voltage region.

(b) Description of the Prior Art

In a scanning electron microscope, a two-dimensional scanning image isobtained in such a way that electrons which have been emitted from anelectron source of a heating type or a field emission type areaccelerated so as to be formed into a slender electron beam (a primaryelectron beam) through an electrostatic or electromagnetic lens, asurface of a sample to be observed is scanned with the resultant primaryelectron beam using a scanning deflector, a secondary signal is detectedwhich is formed by secondary electrons or reflected electrons which aresecondarily generated from the sample by irradiation with the primaryelectron beam, and the intensity of the detected signal is made an inputfor the intensity modulation in a cathode ray tube (CRT) which isscanned synchronously with the scanning of the primary electron beam. Inthe general scanning electron microscope, the electrons which have beenemitted from the electron source are accelerated in a space between theelectron source, to which a negative electric potential is applied, andan anode electrode at ground electric potential, and the surface of thesample to be inspected at ground electric potential is scanned with theelectron beam.

As the result of the fact that the scanning electron microscope hasbecome used in the process of the manufacture of semiconductor devicesor in the inspection after completion thereof (e.g., the sizemeasurement and the inspection of the electrical operation thereof usingthe electron beam), there has been required a high resolution of 10 nmor less at a low acceleration voltage of 1,000V or lower with which aninsulating material can be observed without being charged withelectricity.

The primary factor which impedes attaining of high resolution in the lowacceleration voltage region is the blur of the electron beam due to thechromatic aberration which results from the dispersion in the energy ofthe electron beam emitted from the electron source. In a scanningelectron microscope of the low acceleration voltage type, in order toreduce the blur due to the chromatic aberration, there is mainlyemployed an electron source of a field emission type in which thedispersion in the energy of the emitted electron beam is small. However,even in the case of an electron source of the field emission type, thespace resolution at a voltage of 500V is limited to the range of 10 to15 nm. This does not fulfill the requirement by users.

As for the measure of solving the above-mentioned problem, there isknown a method wherein an acceleration voltage of the primary electronbeam between an electron source and an anode electrode at the groundingelectric potential is set to a value higher than the final accelerationvoltage, and the primary electron beam is decelerated in a space betweenan objective lens at ground electric potential and a sample to beinspected to which a negative electric potential is applied, whereby theacceleration voltage of interest is set to the final low accelerationvoltage (refer to Proceedings of IEEE 9th Annual Symposium on Electron,Ion and Laser Technology, pp. 176 to 186).

The effects which are provided by this method were previously confirmedby the experiments relating thereto. However, that technology has hardlybeen adopted commercially since the secondary electrons are drawn intoan evacuable enclosure through the retarding electric field so that theyare difficult to detect; and a specimen stage having high electricalinsulating characteristics is required because a high voltage is appliedto the sample.

SUMMARY OF THE INVENTION

The present invention provides a breakthrough to the above-mentionedproblems associated with the prior art. According to the presentinvention, the problem associated with detection of the secondary signalis solved by the provision of means for detecting, after it has passedthrough an objective lens, a secondary signal formed by secondaryelectrons, reflected electrons or the like which are extracted into anaperture of the objective lens by an electric field applied across theobjective lens and a sample. In addition, by provision ofpost-acceleration means in an objective lens passage, a negativeelectric potential applied to the sample is reduced down to apracticable value. In such a way, a structure of a scanning electronmicroscope is realized which can be adopted to a commercial system

That is, according to the present invention, there is provided ascanning electron microscope having an electron source, a scanningdeflector by means of which the sample is scanned with a primaryelectron beam generated from the electron source, an objective lens forcondensing the primary electron beam, and a secondary signal detectingunit for detecting a secondary signal which has been generated from thesample by irradiation of the primary electron beam in order to obtain atwo-dimensional scanning image of a sample, the scanning electronmicroscope including an acceleration tube arranged in an accelerationbeam passage of the objective lens, means for applying apost-acceleration voltage of the primary electron beam to theacceleration tube, and means for forming a retarding electric field forthe primary electron beam, in a space between the acceleration tube andthe sample, wherein the secondary signal detecting unit is arranged in aposition closer to the electron source than to the acceleration tube.

According to the present invention, it is possible to solve both theproblem that it is difficult to detect the secondary electrons or thereflected electrons, and the management problem resulting from the factthat a high electric potential is applied to the sample. As a result, itis possible to realize a scanning electron microscope in which thechromatic aberration is reduced in the low acceleration voltage region.

The secondary signal detecting unit may include a conductive targetplate having an aperture through which the primary electron beam passes,extraction means for extracting secondary electrons generated from theconductive target plate, and detection means for detecting the secondaryelectrons extracted into the extraction means. The extraction means isformed by both an electric field and a magnetic field crossing theelectric field so that deflection of the primary electron beam due tothe electric field can be canceled by the magnetic field. The method ofdetecting the secondary signal of this system may also be applied toeither the case where no acceleration tube is provided, or the casewhere the electric potential of the acceleration tube is made 0V (groundelectric potential).

The secondary signal detecting unit may be comprised of either amulti-channel plate having an aperture through which the primaryelectron beam passes, or a photo detector having an aperture, throughwhich the primary electron beam passes, for detecting light emission offluorescent substance.

The installation place of the secondary signal detecting unit may beeither one or both of a position located between the acceleration tubeand a scanning deflector, and a position located between the scanningdeflector and the electron source. In the case where the secondarysignal detecting unit is provided in each of the two positions, thescanning image can be formed using one of the detection signalstherefrom, or the scanning image can be formed by calculating thedetection signals from the respective detectors. Which of methods thescanning image is formed on the basis of may be automatically selectedin correspondence to the magnification of the scanning image or theregiven observation conditions. The method employing the two secondarysignal detecting units may also be applied to either the case where noacceleration tube is provided, or the case where the electric potentialof the acceleration tube is made 0V (ground electric potential).

By combining electrostatic deflection with electromagnetic deflection,the scanning deflector for the primary electron beam can be designed soas to give the desired deflection to the primary electron beam, but notto give a deflection to the secondary signal which has been extractedfrom the sample side. This deflection method wherein electrostaticdeflection is combined with electromagnetic deflection may also beapplied to the case where no acceleration tube is provided.

If both the post-acceleration voltage and the voltage to be applied tothe sample are controlled while maintaining both a ratio of thepost-acceleration voltage to be applied to the acceleration tube to theelectron gun voltage to be applied to the electron source, and a ratioof the voltage to be applied to the sample to the electron gun voltageto be applied to the electron source constant, it is possible tomaintain the cross over point of the secondary signal generated form thesample at a fixed position.

The lens center formed by the magnetic field of the objective lens isaligned with the lens center of the electrostatic lens formed betweenthe acceleration tube and the sample, whereby it is possible to removethe distortion of the scanning image due to the function of theelectrostatic lens formed by the retarding electric field.

If the upper magnetic poles of the objective lens are electricallyinsulated from the remaining portion of the objective lens and thepost-acceleration voltage is applied thereto so that the upper magneticpoles also act as the acceleration tube, the alignment of the lenscenter of the electromagnetic lens with the lens center of theelectrostatic lens can be readily carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects as well as advantages of the presentinvention will become clear by the following description of embodimentsof the present invention with reference to the accompanying drawings,wherein:

FIG. 1 is a schematic view showing a structure of an embodiment of thepresent invention;

FIG. 2 is a schematic view useful in explaining a structure of anembodiment in which a target plate employed therein is used to detect asecondary signal;

FIG. 3 is a schematic view useful in explaining a structure of anembodiment in which a target plate employed therein is used to detect asecondary signal, and an electric field and a magnetic field crossingthe electric field are employed to extract secondary electrons in orderto prevent deflection of primary electrons;

FIG. 4 is a schematic view useful in explaining a structure of anembodiment in which a secondary signal detecting unit is provided abovea scanning deflector;

FIG. 5 is a schematic view useful in explaining a structure of anembodiment in which secondary signal detecting units are respectivelyprovided above and below the scanning deflector;

FIG. 6 is a schematic view, partly in block diagram form, useful inexplaining a structure of an embodiment in which a channel plateemployed therein is used to detect the secondary signal;

FIG. 7 is a schematic view useful in explaining a structure of anembodiment in which a scintillator employed therein is used to detectthe secondary signal;

FIG. 8 is a schematic view useful in explaining a structure of anembodiment in which the intensity of an electric field applied to asample is controlled;

FIG. 9 is a schematic view showing a structure of an example of anembodiment in which a control electrode is provided above the sample;

FIG. 10 is a schematic view showing a structure of an example of acharged particle microscope including a control electrode;

FIG. 11 is a schematic view showing a structure of another example of acharged particle microscope including a control electrode;

FIG. 12 is a schematic view showing a structure of an example of acharged particle microscope in which a control electrode is formed alongan internal face of a sample chamber;

FIG. 13 is a schematic view showing a structure of an example in which apart of a magnetic path of an objective lens is made a controlelectrode;

FIG. 14 is a schematic view useful in explaining a structure of anembodiment in which a scanning deflector is constituted by combinationof an electric field and a magnetic field;

FIG. 15 is a schematic view useful in explaining a structure of anembodiment in which a lens center of an electromagnetic lens is alignedwith the lens center of an electromagnetic lens; and

FIG. 16 is a schematic view useful in explaining a structure of anembodiment in which magnetic poles of a magnetic lens serve as anacceleration tube.

DETAILED DESCRIPTION

Referring first to FIG. 1, there is illustrated a schematic view showinga structure of an embodiment of a scanning electron microscope accordingto the present invention. At the time when an extraction voltage 3 isapplied across a field emission cathode 1 and an extraction electrode 2,electrons 4 are emitted from the cathode 1. The electrons 4 thus emittedare further accelerated (or decelerated in some cases) in a spacebetween the extraction electrode 2 and an anode 5 at ground electricpotential. The energy (the acceleration voltage) of an electron beamwhich has passed through the anode 5 matches an electron gunacceleration voltage 6. In the present invention, the electron beam 7thus accelerated is further post-accelerated through an accelerationtube 9 which is arranged so as to interpenetrate an objective lens 8.The energy of the electron beam when passing through the objective lens8 becomes equal to the sum of the electron gun acceleration voltage 6and a post-acceleration voltage 10 which is applied to the accelerationtube 9. A primary electron beam 11 which has been post-accelerated isdecelerated by a negative superimposed voltage 13 applied to a sample 12so as to obtain a desired acceleration voltage. The substantialacceleration voltage in this method becomes equal to a differencebetween the electron gun acceleration voltage 6 and the superimposedvoltage 13 irrespective of the magnitude of the post-accelerationvoltage 10.

The primary electron beam 7 which has passed through the anode 5 issubjected to the scanning deflection through a condenser lens 14, aupper scanning deflector 15 and a lower scanning deflector 16, and thenis further accelerated by the post-acceleration voltage 10 in theacceleration tube 9 provided through the passage of the objective lens8. A primary electron beam 11 thus post-accelerated is narrowlycondensed onto the sample 12 by the objective lens 8. The primaryelectron beam 11 which has passed through the objective lens 8 isdecelerated by a retarding electric field 17 which is formed in a spacebetween the objective lens 8 and the sample 12 so as to reach thesurface of the sample 12.

By virtue of this structure, the acceleration voltage of the primaryelectron beam when passing through the objective lens 8 becomes higherthan the final acceleration voltage. As a result, as compared with thecase where the primary electron beam having the final accelerationvoltage passes through the objective lens 8, the chromatic aberration inthe objective lens is reduced, and hence a narrower electron beam (highresolution) can be obtained. An opening angle of the primary electronbema in the objective lens 8 is determined by a diaphragm 18 which isarranged below the condenser lens 14. The operation of centering thediaphragm 18 is carried out by adjusting an adjustable lug 19. In thisconnection, while the mechanical adjustment is carried out using theadjustable lug 19 in the figure, alternatively, there may be adopted ameasure wherein electrostatic or electromagnetic deflectors arerespectively provided before and after the diaphragm 18 so as tosuitably deflect the electron beam for adjustment of the centeringthereof.

The surface of the sample 12 is scanned with the electron beam which hasbeen narrowly condensed by the objective lens 8 by both the upperscanning deflector 15 and the lower scanning deflector 16. In thisconnection, both the deflection direction and the intensity of each ofthe upper scanning deflector 15 and the lower scanning deflector 16 areadjusted in such a way that the scanning electron beam always passesthrough the center of the objective lens 8. The sample 12 is mounted toa sample holder 20 to which a superimposed voltage 13 is applied. Thesample holder 20 is placed on a specimen stage 22 through an insulatingstage 21 so that the horizontal position of the sample holder 20 can besuitably adjusted.

The sample 12 is irradiated with the primary electron beam 11, therebygenerating secondary electrons 23. Since a retarding electric field 17which is formed in a space between the objective lens 8 and the sample12 serves as an acceleration electric field for the secondary electrons23, the secondary electrons 23 are extracted into the passage of theobjective lens 8 and rise upwardly while being influenced thereupon bythe lens function established by the magnetic field of the objectivelens 8. The secondary electrons 23 which have passed through theobjective lens 8 are extracted by the electric field in the transversedirection of an extraction electrode 24 which is arranged between theobjective lens 8 and the lower scanning deflector 16. After havingdiffused through a mesh of the extraction electrode 24, the secondaryelectrons 23 are accelerated by a scintillator 25 to which 10 kV (apositive electric potential) is applied so as to cause the scintillator25 to emit light. The light thus emitted is introduced through lightguide 26 into a photo multiplier (PM) 27 which converts it into anelectrical signal. An output of the photo multiplier 27 is furtheramplified to be an intensity modulation input to a cathode ray tube(CRT)(not shown).

The feature of this structure is that the acceleration voltage of theelectron beam when passing through the condenser lens 14, the diaphragm18 and the objective lens 8 is higher than the final energy. Inparticular, it is higher while passing through the objective lens 8. Soas to control the chromatic aberration in this course, the electron beamis further post-accelerated. In a typical example, when the electron gunacceleration voltage, the post-acceleration voltage, and the negativesuperimposed voltage applied to the sample 18 are 1,000V, 1,000V and500V, respectively, the substantial acceleration voltage is 500V. Thechromatic aberration is reduced by about 50% since the accelerationvoltage is 2,000V when the electron beam is passing through theobjective lens 8. In the case where the acceleration voltage is 500V,the beam diameter (resolution) which was 15 nm in the former case isimproved down to 7 nm.

In the above-mentioned embodiment, the secondary electrons 23 areextracted to the outside of the electron beam passage by the extractionelectrode 24 so as to be detected. In this method, the energy of thesecondary electrons 24 is increased as the superimposed voltage 13 isfurther increased, and therefore, the voltage to be applied to theextraction electrode 24 needs to be correspondingly increased. As aresult, there arises a problem that the primary electron beam (theaccelerated electron beam 7) is also deflected.

An embodiment employing a target plate as shown in FIG. 2 solves theabove-mentioned problem and makes the detection of high efficiencypossible. In the present embodiment, there is provided a target plate 29having a central hole 28 in the electron beam passage. The surface ofthe target plate 29 is coated with a material, such as gold, silver orplatinum, from which the secondary electrons are readily generated bythe electron irradiation. After having passed through the central hole28 of the target plate 29, the accelerated primary electron beam 7enters into the acceleration tube 9. The diameter of the central hole 28is set to a value with which the electron beam which has been deflectedby both the upper and lower scanning deflectors 15 and 16 has nocollision with the target plate 29. The secondary electrons 23 whichhave been generated from the sample 12 and then are accelerated by thesuperimposed voltage 13 pass through the acceleration tube 9 while beingcaused to diverge by the lens function of the objective lens 8, and thencollide with the rear face of the target plate 29. Although different inorbit from the secondary electrons, likewise, the reflected electronswhich have been generated from the sample 12 collide with the rear faceof the target plate 29.

Secondary electrons 30 which have been generated from the rear face ofthe target plate 29 are extracted by the electric field provided by theextraction electrode 24, and then similarly to the case shown in FIG. 1converted into an electrical signal via the scintillator 25, the lightguide 26 and the photomultiplier 27. The feature of this structure isthat even if the superimposed voltage 13 applied to the sample is highand hence the acceleration for the secondary electrons 23 is increased,since the object of detection is the secondary electrons 30 which havebeen generated from the target plate 29 and are not accelerated, thevoltage applied to the extraction electrode 24 may be low. For thisreason, it is possible to reduce the influence of the electric fieldprovided by the extraction electrode 24 upon the primary electron beam7. While the scintillator 25 is used to detect the extracted secondaryelectrons in this case, alternatively, an electron detection amplifiersuch as a channel plate or a multi-channel plate may instead beemployed.

Referring to FIG. 3, there is shown an example wherein a magnetic fieldB is applied so as to cross an electric field E for extracting thesecondary electrons 30 which have been generated from the target plate29. By adopting this structure, it is possible to correct the deflectionof the primary electron beam 7 provided by the above-mentionedextracting electric field E. In other words, the deflection of theprimary electron beam 7 provided by the extracting electric field E canbe corrected by the deflection provided by the magnetic field B. Now,reference numerals 31' and 31" designate electric field deflectingelectrodes for generating the extracting electric field, respectively.One electric field deflecting electrode 31' is formed of a mesh so thatthe secondary electrons 30 can diffuse therethrough. Reference numerals32' and 32" designate transverse magnetic field deflecting coils (thecoils 32' and 32" for generating the magnetic field B are symbolicallyillustrated in the figure). The magnetic field B which is formed by thetransverse magnetic field deflecting coils 32' and 32" crosses theelectric field E, and the intensity of the magnetic field B is adjustedso as to cancel the deflection provided by the electric field E which isapplied to the accelerated electron beam 7. While a pair of transversemagnetic field deflecting coils 32 are employed in the presentembodiment, if two pairs of transverse magnetic field deflecting coilsare employed which are arranged so as to form a suitable angletherebetween, the degree of orthogonality between the electric field andthe magnetic field can be rigidly adjusted by adjusting the currentwhich is caused to flow through each pair, or the like. It is to beunderstood that even if instead of providing the two pairs of transversemagnetic field deflecting coils 32, two pairs of electric fielddeflecting electrodes are provided in order to adjust the direction ofthe electric field, likewise, the degree of orthogonality between theelectric field and the magnetic field and be rigidly adjusted.

Incidentally, the secondary signal detecting method employing the targetplate 29 shown in FIGS. 2 and 3 is available to either the case where noacceleration tube 9 is provided, or the case where the acceleration tube9 is grounded.

Referring to FIG. 4, there is illustrated an embodiment wherein thesecondary signal detecting unit is provided above the upper scanningdeflector 15. In the figure, the secondary signal detecting unit isprovided between the upper scanning deflector 15 and the diaphragm 18.While similarly to the case of FIG. 2, the target plate 29 has thecentral hole 28 bored therethrough, since the primary electron beam isnot yet subjected to the scanning deflection in this position, thediameter of the central hole 28 may be the same as that of the diaphragm18 for limiting an angular aperture of the minimum primary electronbeam. In the present embodiment in the figure, the target plate 29having a central hole 28 with 0.1 mm diameter is arranged below thediaphragm 18. In this connection, the diaphragm 18 may also serve as thetarget plate 29.

In the case where the target plate 29 was arranged below the scanningdeflector, the diameter of the central hole 28 of the target plate 29was set to a value such that the deflected electron beam does notcollide with the target plate 29. Comparing the diameters of the centralholes 28 with each other with respect to the respective typicalexamples, a diameter of 3 to 4 mm is required in the case where thetarget plate 29 is arranged below the scanning deflector, while adiameter of 0.1 mm or smaller may be available in the case where thetarget plate 29 is arranged above the scanning deflector. Thus, since ifthe target plate is arranged above scanning deflector, the central holeof the target plate can be made sufficiently small, the efficiency ofcapturing the secondary electrons by the target plate is necessarilyimproved.

In the present embodiment shown in FIG. 4, the sample 12 is placedwithin a lens gap of the objective lens 8. This arrangement contributesto the reduction of the chromatic aberration coefficient of theobjective lens 8, and has a shape for seeking higher resolution. Thespecimen stage 22 is also provided within the objective lens 8.

Referring to FIG. 5, there is illustrated an embodiment wherein thesecondary signal detecting units are provided in both the upper positionand the lower position of the scanning deflector, respectively. That is,a upper detector 33 is provided above the upper scanning deflector 15,while a lower detector 34 is provided between the lower scanningdeflector 16 and the acceleration tube 9. As shown in FIGS. 3 and 4, theupper detector 33 and the lower detector 34 include the target plates29a and 29b, the electric field deflecting electrodes 31a and 31b, thetransverse magnetic field deflecting coils 32a and 32b, thescintillators 25a and 25b, the light guides 26a and 26b, and the photomultipliers 27a and 27b, respectively.

In the present embodiment, the secondary electrons or the reflectedelectrons which have passed through the central hole 28b of the targetplate 29b of the lower detector 34 can be detected by the upper detector33. Since the secondary signal which is detected by the upper detector33 contains a large number of secondary electrons and reflectedelectrons which have been emitted from the sample 12 perpendicularlythereto, an image can be obtained which is different in the contrastfrom that in the lower detector 34. For example, in the inspection ofthe contact holes in the process of manufacturing a semiconductordevice, if the lower detector 34 is used, an image having a contact holeimage portion which is emphasized from the periphery thereof can beobtained, while if the upper detector 33 is used, the detailed image ofthe bottom of the contact hole can be obtained In addition, the contrastin which the features of the sample is emphasized can be formed bycalculating the signals output from the detectors 33 and 34,respectively.

While which of the outputs from the upper and lower detectors thescanning image is formed from can be selected by an operator,alternatively, it may be automatically selected under predeterminedconditions For example, in the case where the observation magnificationis 2,000 or lower, the lower detector 34 is selected, while in the casewhere the observation magnification is 2,000 or higher, the upperdetector 33 is selected. In addition, which of the upper and lowerdetectors should be used may be determined in correspondence to thesample to be observed. In this case, the procedure of inputting datarelating to the kind of sample to be observed to the system, and soforth is carried out. For example, when the data relating to theobservation of the contact hole of a semiconductor device is input, theupper detector 33 with which the image of the inside of the contact holeis emphasized is automatically selected, while when a photo resist filmapplied to the surface of the substrate body is observed, the lowerdetector 34 is selected.

Incidentally, in the embodiment shown in FIG. 4 or FIG. 5, even if theacceleration tube 9 is removed or the acceleration tube 9 is grounded,the effect associated therewith is great and hence such a case issufficiently practical.

Referring to FIG. 6, there is shown an embodiment wherein the secondarysignal is detected using a multi-channel plate detector. A multi-channelplate detector body 35 has a disc-like shape and has a central hole 28through which the primary electron beam is to pass. In addition, a mesh37 is provided below the multi-channel plate detector body 35 and isgrounded. With such a structure, after having passed through the centralhole 28 of the multi-channel plate, the accelerated primary electronbeam 7 is condensed by the objective lens so as to be applied to thesample. The secondary electrons 23 which have been generated from thesample pass through the mesh 37 to be made incident on the channel platedetector body 35. The secondary electrons 23 which have been madeincident on the channel plate detector body 35 are accelerated andamplified by an amplification voltage 38 which is applied across boththe end portions of the channel plate detector body 35. The electrons 39which have been amplified are further accelerated by an anode voltage 40to be captured by an anode 41. After having been amplified by anamplifier 42, the secondary electron signal thus captured is convertedinto an optical signal 44 by an optical conversion circuit 43. Thereason for converting the secondary electron signal into an opticalsignal 44 is that the amplifier 42 is in a floating state due to theamplification voltage 38 of the channel plate body 35. The opticalsignal 44 is converted into an electrical signal again in an electricalconversion circuit 45 at ground electric potential so as to be utilizedas the intensity modulation signal of the scanning image.

Now, the anode 41 may be divided into two or four portions in order toobtain information concerning the emission direction of the secondaryelectrons 23. In this case, it is to be understood that the number ofamplifiers 42, optical conversion circuits 43 and electrical conversioncircuits 45 corresponding to the division number is required, and alsosignal processing calculating the divided signals is carried out.

Referring to FIG. 7, there is shown an embodiment wherein the secondarysignal is detected utilizing a monocrystalline scintillator. In FIG. 7,a monocrystalline scintillator 46 is, for example, constructed in such away that a cylindrical YAG monocrystal is cut diagonally and an aperture47 through which the primary electron beam passes is bored through thecutting plane thereof. In this connection, the head portion of thescintillator 46 is coated with a conductive film 48, such as metal orcarbon, which is grounded. The secondary electrons 23 which have beengenerated from the sample 12 are applied to the scintillator 46, therebycausing the scintillator 46 to emit light. The light thus emitted isreflected from the diagonal portion to be introduced into thephotomultiplier 27 through the light guide formed by the cylindricalportion so as to be detected and amplified therein. Incidentally, whilein the present embodiment, both the light emitting portion and the lightguide of the scintillator 46 are made of YAG monocrystal, the presentinvention is not limited thereto. That is, alternatively, a structuremay be adopted in which only the light emitting portion for detectingthe secondary electrons is made of YAG monocrystal or fluorescentsubstance, and also the light guide is made of a transparent body suchas glass or resin.

Next, the description will hereinbelow be given with respect to acontrol method for carrying out efficient detection of the secondarysignal with reference to FIG. 7. Since the secondary signal (e.g, thesecondary electrons) passes through magnetic field of the objective lens8, the secondary signal undergoes the lens function, and as a result, across-over 49 of the secondary electrons is formed. If the secondaryelectrons are focused at the aperture 47 of the scintillator 46 due tothe lens function, almost all the secondary electrons pass through theaperture 47 and hence they can not be detected. Then, the secondaryelectrons are adjusted so as to be focused at the position before orafter the target plate in order to enhance the detection efficiency. Inthe present embodiment, both the post-acceleration voltage and thesuperimposed voltage applied to the sample are controlled so as toprevent the focusing position of the secondary electrons to be changedwith changing acceleration voltage (the substantial accelerationvoltage).

When the current which is caused to flow through the lens coil is I, thenumber of turns of the coil is N, and the acceleration voltage of theelectron when passing through the lens magnetic field is V, the focallength of the electromagnetic lens is a function of a variable I N/V1/2. When the electron gun acceleration voltage is Vo and thepost-acceleration voltage applied to the acceleration tube is Vb, theacceleration voltage when the primary electrons pass through the lensmagnetic field is expressed by (Vo+Vb). Since the sample position (thefocal length) is fixed, I NI (Vo+Vb) 1/2 always becomes a constant value(=a). When the superimposed voltage applied to the sample is Vr, theacceleration voltage when the secondary electrons pass through the lensmagnetic field is expressed by (Vr+Vb),and a variable I N/V 1/2 isexpressed by the following expression:

    I N/V 1/2=a(Vo+Vb) 1/2/(Vr+Vb) 1/2=a{1+(Vb/Vo)}1/2/{(Vr/Vo)+(Vb/Vo)}1/2

From this expression, it will be understood that if the ratios of Vr/Voand Vb/Vo are controlled so as to be constant, the focal length of thesecondary electrons becomes constant. That is, if both thepost-acceleration voltage Vb and the superimposed voltage Vr applied tothe sample are controlled with the ratios of Vr/Vo and Vb/Vo madeconstant, the focal position of the secondary electrons can becontrolled so as to be constant without depending on the accelerationvoltage the substantial acceleration voltage).

Referring to FIG. 8, there is shown an example wherein a controlelectrode for controlling the electric field applied to the surface ofthe sample is additionally provided. As apparent from the figure, acontrol electrode 36 is provided between the objective lens 8 and thesample 12, and a control voltage 50 is applied to the control electrode36. The control electrode 36 has a hole through which the electron beampasses. The strength of the electric field which is formed between theacceleration tube 9 and the sample 12 and is applied to the sample 12 iscontrolled by the control electrode 36. This structure is effective inthe case where it is inconvenient when the electric field of highstrength is applied to the sample. For example, this inconvenient caseresults from the problem that the elements will be damaged if a highelectric field is applied to the wafer having a semiconductor integratedcircuits formed therein.

In addition, this structure is effective in the specific case where whenthe periphery of the wafer is coated with an oxide film, an electricalcontact can not be made successfully between the wafer and the sampleholder 20. More specifically, this occurs in the case where both theside face and the rear face of the sample (the wafer) are covered withthe insulating material and an electrical connection for retarding cannot be carried out. In addition, that reason is that in the case wherethe sample (wafer) 12 is placed within the electric field which isgenerated between the sample holder 20 and the objective lens 8 andhence no control electrode is provided, since only the intermediateelectric potential between the superimposed voltage 13 applied to thesample holder 20 and the grounding electric potential of the objectivelens 8 is applied to the sample 12, the normal observation an not becarried out.

In addition, the electric potential at the control electrode 36 is madeeither the same as that of the sample holder 20 to which thesuperimposed voltage 13 is applied or higher than that of the sampleholder 20 by positive several tens of volts, whereby it is possible toprevent any element from being damaged and also it is possible toprevent the electric potential of the wafer from floating from theelectric potential of the sample holder 20. In this case, the controlelectrode 26 needs to be made large enough to always cover the wholesample (wafer)

Referring to FIG. 9, there is illustrated a schematic view showing astructure of an example in the case where a control electrode isadditionally provided.

A control electrode 60 having an aperture 59 through which the primaryelectron beam passes is provided above the sample (wafer) 12. The samevoltage as the superimposed voltage 13 applied to the sample holder 20is applied to the control electrode 60. When the control electrode 60 atthe same electric potential as that of the sample holder 20 is arrangedabove the sample (wafer) 12, the wafer is necessarily surrounded by themetallic members at the same electric potential and hence the electricpotential of the wafer becomes the same as that of the surroundingmetallic members. Strictly speaking, the electric potential of the waferis slightly different as an error from that of the surrounding metallicmembers due to invasion of the electric field into the aperture 59through which the primary electron beam passes. This error correspondsapproximately to the ratio of the area of the aperture 59 to the areasof the sample (wafer) 12. For example, when the wafer is 8 inches indiameter and the aperture 59 is 10 mm in diameter, the area ratio is1/400 and hence the error in the electric potential is only 1% Thisvalue is sufficiently small.

In the above-mentioned structure,, the same electric potential as thatof the metallic members surrounding the wafer can be applied to thewafer.

As a result, even in the case where both the front face and the rearface of the wafer are coated with the electrically insulating film andhence the wafer can not be electrically connected to the specimen stageor the like, the voltage for retarding can be applied.

In this embodiment, within the sample holder 20, by structuring at leastthe portion located under the sample (wafer) 12 with a conductor forapplying the superimposed voltage, then the wafer is covered with ametal of the same potential. The sample holder itself might be aconductor, or a conductor might be inserted into the sample holder.

Referring to FIG. 10, there is illustrated a schematic view showing astructure of another example wherein the control electrode isadditionally provided.

The control electrode 60 is provided between the sample (wafer) 12 andthe objective lens 8. The same voltage as the superimposed voltage 13applied to the sample holder 20 is applied to the control electrode 60.As a result, the sample (wafer) 12 is surrounded by both the sampleholder 20 and the control electrode 60 to which the same voltage isrespectively applied. Then, as described above, even when the sample(wafer) 12 is coated with the electrically insulating film, the samevoltage as the superimposed voltage 13 can be substantially applied tothe sample (wafer) 12.

The aperture 59 of the control electrode 60 normally has a circularshape. However, the aperture may have any other shape other than acircular shape. In addition, the aperture 59 needs to have a size withwhich the field of view to be observed is not hindered. In the presentembodiment, the aperture 59 is 4 mm in diameter. Since a space betweenthe control electrode 60 and the sample (wafer) 12 is 1 mm, there is afield of view with 4 mm diameter. In addition, since the retardingelectric field reaches the wafer through the aperture 59, the secondaryelectrons can be effectively made to rise above the objective lens 8.When the diameter of the aperture is decreased, the retarding electricfield does not reach the sample (wafer) 12. However, in the case wherethe wafer needs to be inclined, or in the case where the sample hasirregularity, such a condition is suitable therefor and hence it ispossible to reduce occurrence of astigmatism and deviation in field ofview.

The stage 22 is provided in order to observe an arbitrary position onthe sample (wafer) 12. Now, when a position which deviates largely fromthe center point of the sample (wafer) 12 is an object of observation,the sample (wafer) 12 needs to be largely moved. In this connection, ifthe sample (wafer) 12 is positioned outside the control electrode 60,the electric potential of the sample (wafer) 12 is changed and hence thefixed retarding voltage can not be applied thereto.

In order to cope with this situation, in the present embodiment, thecontrol electrode is formed along the movement orbit of the sample(wafer) 12. By adopting this structure, even if the position of sample(wafer) 12 is changed due to the movement of the specimen stage 22, afixed retarding voltage can be applied, and also it is possible toprevent any element from being damaged due to application of theelectric field generated between the objective lens 8 and the sample(wafer) 12 thereto.

In addition, in the present embodiment, it is preferable to arrange thecontrol electrode having a size over the movement range of the wafer.More specifically, the diameter of the control electrode which is usedto observe the overall surface of the 8-inch wafer is set to 400 mm. Byadopting such a structure, however the wafer may be moved, the voltageapplied to the wafer can be maintained constant.

Incidentally, while the control electrode has a flat plate shape in thepresent embodiment, the present invention is not limited thereto. Thatis, the control electrode is formed so as to have a mesh-like shape or ashape having a large number of holes or slits formed therein, wherebythe vacuum exhaust property can be improved. In this case, the diameterof the hole or the width of the slit is preferably smaller than thespace between the wafer and the control electrode.

In FIG. 10, at the time when the primary electrons 63c have passedthrough the aperture 59 of the control electrode 60 and are applied tothe sample (wafer) 12, the secondary electrons 62 are generated from thesame (wafer) 12. The secondary electrons 62 thus generated areconversely accelerated by the retarding electric field to the primaryelectrons 63c so as to be introduced above the objective lens 8. In thisconnection, since the secondary electrons 62 undergo a lens function dueto the magnetic field of the objective lens 8, the secondary electrons62 are introduced above the objective lens 8 while forming a focal pointas shown in the figure.

The secondary electrons 62 thus introduced collide with the target plate29, thereby generating secondary electrons 30. These secondary electrons30 are deflected by an electric field generated by a deflectionelectrode 31' to which a negative electric potential is applied and adeflection electrode 31" to which a positive electric potential isapplied, the deflection electrodes 31' and 31" being arranged so as tobe mutually opposite to each other. Since the deflection electrode 31"is formed of a mesh, the secondary electrons 30 pass through the mesh tobe detected by the scintillator 25. Reference numerals 32' and 32"designate deflection coils, respectively, which generate a magneticfield crossing the electric field generated by both the deflectionelectrodes 31' and 31", and serve to cancel the deflection functioninfluencing the primary electron beam 63b provided by both thedeflection electrodes 31' and 31".

Incidentally, the control electrode 64 is cooled by cooling means (notshown), thereby enabling contamination, which is provided by scanningthe surface of the sample with the primary electron beam 63c.

Referring to FIG. 11, there is illustrated a schematic view showing astructure of still another example in the case where the controlelectrode is additionally provided.

The elements such as the field emission cathode 11 the extractionelectrode 2, the anode 5, the condenser lens 4, the objective lens 8,the sample 12, the sample holder 20, the electrical insulating stage 21and the specimen stage 22 are accommodated in an evacuable enclosure 66.Incidentally, the illustration of a vacuum exhaust system is omittedhere for the sake of simplicity Now, in the state in which the negativesuperimposed voltage is applied to the sample 12, it should be avoidedthat the work of exchanging the sample is carried out by a sampleexchange mechanism 67, and the evacuable enclosure 66 is made atatmospheric pressure. In other words, only when the surface of thesample 12 is scanned with the electron beam, need the superimposedvoltage 13 be applied to the sample 12.

Then, in the present embodiment, only when all of a first condition inwhich as the preparation operation in attaching/exchanging the sample, aswitch 68 is closed so as to apply the acceleration voltage 6, a secondcondition in which both valves 69 and 70 are opened which are providedbetween the field emission cathode 1 and the sample 12, and a thirdcondition in which a valve 71 is closed through which the sampleexchange mechanism 67 passes in order to place the sample 12 on thespecimen stage 22 are fulfilled, is the control carried out in which aswitch 72 is closed so as to apply the superimposed voltage 13 to thesample 12.

In addition, the sample holder 20 is electrically connected to thespecimen stage 22 through a discharging resistor 73 so that at the timewhen the switch 37 is opened, the electric charges accumulated in thesample 12 are speedily discharged at a fixed time constant through thesample holder 20, the discharging resistor 73 and the specimen stage 22(which is grounded) so as to decrease the electric potential of thesample 12. The discharging resistor may also be self-contained in powersource of the superimposed voltage 13.

According to the apparatus in a preferred embodiment of this invention,a sequence is incorporated in which the acceleration voltage can beapplied under condition in which the degree of vacuum in the peripheryof the field emission cathode 1 is less than a predetermined value, andalso valves 69 and 70 are opened only when the degree of vacuum in theevacuable enclosure is more than a predetermined value. According toutilization of such sequence, an accident such as irradiating electronbeam by a human error irrespective that preparation for irradiatingelectron beam has not been established.

In addition, in the present embodiment, the description has been givenin which only when all the above-mentioned three conditions arefulfilled, the superimposed signal 13 is applied. However, the presentinvention is not limited thereto. That is, even when one or two of theconditions are fulfilled, the switch 72 may be closed.

FIG. 12 shows another embodiment of an arrangement having a controlelectrode added thereto This is an application of the invention to ascanning microscope having a sample stage that enables a sample to betilted. In this embodiment, the control electrode 76 is mounted to thescanning microscope so that it may cover the top of the sample 75. Fromanother viewpoints the control electrode 76 is located along the shapeof the objective lens. The objective lens is shaped so that it does notobstruct the movement of the sample 12. As shown in FIG. 12, if theapparatus provides a tilting device, the objective lens is shaped keenlytoward the sample 12. The location of the control electrode along theobjective lens so shaped results in keeping a smooth movement of thesample.

This arrangement allows the sample (wafer) 12 to be enclosed by thesample holder 20 and the control electrode 76 irrespective of theposition and the inclination of the sample (wafer) 12. This arrangementdisallows an electric field to be generated on the surface of the sample(wafer) 12. A numeral 20a denotes the state where the sample (wafer) 12is inclined. A numeral 74 denotes an inclining mechanism built in asample stage 22. In this embodiment, it is preferable that the aperture65 of the control electrode 76 has a smaller diameter than a distancebetween the aperture 65 and the sample 12. For improving the sensingefficiency of secondary electrons, the voltage to be applied to thecontrol electrode 76 is shifted by a few tens of positive voltagesrather than the voltage to be applied to the sample 12.

In order to apply a retarding voltage onto the sample, it is preferableto set each voltage for the sample or the control electrode so that adesirous voltage is applied to the sample. This setting of each voltageto the sample or the control electrode is a result of considering theimpact of the compound electric fields derived by the voltages appliedonto the sample and onto the control electrode.

Further, the aperture may be made larger in diameter. The largeraperture allows an electric field for extracting the secondary electronsto be applied onto the sample 12. In this case, the inclined samplecauses the observation position to be shifted. The adverse effect givenby this shift may be eliminated by measuring the inclination and theshift in advance and deflecting the electron beam accordingly orcorrecting the specimen stage 22 such as by horizontal movement of thestage 22. The control electrode 76 is made of a nonmagnetic materials sothat it does not have an adverse effect on the characteristics of theobjective lens 8.

In this embodiment, the control electrode is located so that it coversthe interior of the specimen chamber. This location is not necessary.The control electrode is just required to be shaped along the movementof the sample. Even this minimum shape allows the sample to be envelopedby the sample holder and the control electrode. In the foregoingdescription, the sample holder has been described as a conductorspecified in the invention of the present application. In place, aconductor may be located on or under the sample holder. In the foregoingembodiments, a conductor that is larger than the sample allows thesample (wafer) to be substantially enveloped by the control electrodeand the conductor, thereby making it possible to apply a constantretarding voltage to the sample.

FIG. 13 shows another arrangement of the scanning microscope where acontrol electrode is mounted. Herein, the control electrode is notgrounded between the objective lens 8 and the sample 12. The objectivelens 8 is constructed of an excitation coil 78, an upper magnetic path77 and a lower magnetic path 79 so that the lower magnetic path 79located opposed to the sample 12 is electrically insulated from theupper magnetic path 77 and the overlapping voltage 13 is applied to thelower magnetic path 79. In place, the potential to be applied to thelower magnetic path 79 may be derived as a positive potential from thesample 12. This makes it possible to efficiently conduct the secondaryelectrons onto the objective lens 8.

FIG. 14 is an explanatory view showing a scanning deflector for anelectron beam that uses a combination of an electric field and amagnetic field. If a secondary electron detector is mounted on thescanning deflector, the scanning deflector acts to deflect the secondaryelectrons generated from the sample when those electrons passes throughthe deflector itself. At a low magnification where the electronic beamhas a larger angle of scanning deflection, the secondary electrons aredeflected more, so that those electrons may rush against the inner wallof a beam passage, which may obstruct the sensing of the secondaryelectrons The arrangement of this embodiment is intended to overcomethis shortcoming The scanning deflector is constructed of eight (8)polar electrostatic deflectors 51a to 51h and magnetic field deflectors51a to 51h.

Now, consider the x-axial deflection. The application of a positivepotential to the electrodes 51h, 51a and 51b of the 8 pole electrostaticdeflectors and a negative potential to the electrodes 51a, 51e and 51fresults in yielding a deflected electric field Ex. As shown in FIG. 14,a potential of Vx is applied to the electrodes 51a and 51e and apotential of a half of Vx is applied to the electrodes 51h, 51b, 51d and51f located on both sides of the line-drawn between the electrodes 51aand 51a. This is a well-known technique of yielding a uniform electricfield. Concurrently with the electric field, current Ix is passedthrough the coils 52a and 52c of the magnetic deflector 52 so that amagnetic field Mx is generated in the orthogonal direction to themagnetic field Ex as shown in FIG. 14. This electric field Ex and themagnetic field Et act in concert to cancel the deflection developed forthe secondary electrons coming form below or magnify the primaryelectrons fired from above.

The deflection θ(S) developed for the secondary electrons coming frombelow is a difference between a deflection θ(B) caused by the magneticfield and a deflection θ(E) caused by the electric field, which isarithmetically represented as follows. ##EQU1## where L denotes anactive distance of the electric field and the magnetic field, e and mdenotes a charge and a mass of electrons, and Vr denotes an accelerationvoltage given when the secondary electrons pass through the scanningdeflector. Since the ratio of Ex to Bx is represented by the followingexpression, no deflection is developed for the secondary electronscoming from below.

    Bx/Ex=(2m/e).sub.1/2 /8Vr.sub.1/2

On the other hand, the application of the magnetic field deflection tothe electric field deflection enables the deflection of the primaryelectrons to be represented as follows. ##EQU2## where Vo is a primaryacceleration voltage.

Hence, in the condition that no deflection takes place about thesecondary electrons, the angle of deflection θ(o) is represented asfollows.

    θ(o)=(e/2m).sub.1/2 B×L{1+{Vr/Vo).sub.1/2}/ Vo.sub.1/2

The foregoing description has been oriented to the x-axial deflection.The description holds true to the y-axial deflection. That is, bysetting the potential of the electrode 51c as Vy, the potentials of theelectrodes 51b and 51d as Vy/2_(1/2), the potential of the electrode 51gas Vy, and the potentials of the electrodes 51f and 51h as -Vy2_(1/2), ay-axial deflected electric field Ey is obtained. At a time, by passingthe current Iy through the coils 52b and 52d of the magnetic fielddeflector, the magnetic field By crossed with the electric field Ey isobtained. Like the description above, the electric field Ey and themagnetic field By act in concert to cancel the deflection developed forthe secondary electrons coming from below or strength the primaryelectrons coming from above.

In actuality, the deflection is a combination of the x-axial deflectionand the y-axial deflection. Hence, the potential of each deflectionelectrode is an addition of the x-axial deflecting potential as shown inFIG. 14. The actual apparatus is constructed to have two stages of thedeflector, that is, an upper and a lower scanning deflectors so that thedeflected primary electrons are allowed to pass through the center ofthe objective lens. The potentials Vx and Vy of the deflection electrodeand the deflection coil currents Ix and Iy are changed with time whilekeeping the foregoing relation, for realizing the desired scanning ofthe primary electron beam onto the sample.

In turn, the description will be oriented to the relation between thelens center of the magnetic field type objective lens and the lenscenter of the electrostatic lens formed between the acceleration tubeand the sample. FIG. 15A is an explanatory view showing a problem causedif the center CB of the objective lens 8 does not coincide with thecenter CE of the electrostatic-static lens formed between theacceleration tube 9 and the sample 12. In this case, the primaryelectron beam 11 accelerated at a post stage is deflected so that thebeam 11 passes through the center CB of the magnetic field lens but itshifted only by a distance d from the center CE of the electrostaticlens. The larger shifting distance d results in adding a sphericalaberration to the lens effect of the electrostatic lens, therebydistorting the scan image.

FIG. 15B shows the arrangement where the lens center CB of the magneticfield type objective lens 8 coincides with the lens center CE of theelectrostatic lens formed between the acceleration tube 9 and the sample12. In this embodiment, an upper magnetic pole 53 of the objective lens8 is projected opposed to the sample 12, so that the magnetic field isproduced in a space between the sample 12 and the acceleration tube 9where the electrostatic lens is formed, for making the centers of bothlenses coincide with each other. As a result, no lens effect of theelectrostatic lens is applied on the primary electron beam 11accelerated at a post stage. Hence, the resulting scan image is notdistorted.

FIG. 16 shows a structure of the objective lens 8 for more effectivelyrealizing the coincidence of the centers between the magnetic field lensand the electrostatic lens. In the foregoing illustrative embodiments,the acceleration tube 9 is inserted into the electron beam passage ofthe objective lens 8. In this construction, the shift of the axialcenter between the electrostatic lens produced by the acceleration tubeand the magnetic field lens produced by the objective lens results inlowering resolution For avoiding this unfavorable state, it is necessaryto precisely keep the mechanical centers of the lenses in alignment witheach other, This embodiment is concerned with this point. The uppermagnetic pole 53 is projected to the end level of the lower magneticpole 54 and faces the sample 12. The upper magnetic pole 53 iselectrically insulated from the remaining portion of the objective lensby an insulating plate 55. The voltage 10 accelerated at a post stage isapplied to the upper magnetic pole 53.

According to this embodiment, the upper magnetic pole for defining thelens center of the objective lens 8 is shared with the post-accelerationelectrode. This does not bring about any shift between the electrostaticlens and the magnetic field lens. Further, the upper magnetic pole 53 ofthe magnetic field lens is directly opposed to the sample 12 and thepost-acceleration voltage is applied to the upper magnetic pole 53. Thismakes it possible to keep the lens centers of the electrostatic lens andthe magnetic field lens in alignment with each other.

What is claimed is:
 1. A scanning microscope for producing an image of asample and having an electron source, scanning deflectors for scanning aprimary electron beam fired from said electron source onto said sample,an objective lens for converging said primary electron beam, an electrondetector for detecting electrons formed from said sample, wherein:aconductive target plate having an aperture for passing said primaryelectron beam is located between said objective lens and said electronsource, and a deflecting electrode for deflecting said electrons islocated between said objective lens and said target plate.
 2. A scanningmicroscope according to claim 1, wherein said target plate is a flatpanel including an aperture for passing through said primary electronbeam.
 3. A scanning microscope according to claim 1, further comprisinga magnetic coil for generating a magnetic field in a direction crossingthe electric field formed by said deflection electrode.
 4. A scanningmicroscope according to claim 3, wherein said magnetic field formed bysaid magnetic coil acts to control deflection of said primary electronbeam.
 5. A scanning microscope according to claim 1, further comprisinga negative voltage applying power source for applying a negativepotential to said sample.
 6. A scanning microscope according to claim 1,wherein a second electron detector is provided between said objectivelens and said scanning deflector.
 7. A scanning microscope according toclaim 1, wherein a second electron detector is provided between saidscanning deflector and said electron source.
 8. A scanning microscopeaccording to claim 1, wherein said scanning deflector is served throughthe effect of a combination of electrostatic deflection and magneticfield deflection and is adjusted to apply desirous deflection to saidprimary electron beam but no deflection to said electrons extracted fromsaid sample.
 9. A scanning microscope according to claim 1, furtherincluding means for applying a negative voltage onto said sample.
 10. Ascanning microscope according to claim 1, wherein an acceleration tubeis disposed in an electron beam passage of said objective lens, and saidacceleration tube further includes means for providing voltage foraccelerating said primary electron beam.
 11. A scanning microscope forproducing an image of a sample and having an electron source, scanningdeflectors for scanning a primary electron beam fired from said electronsource onto said sample, an objective lens for converging said primaryelectron beam, and an electron detectors for detecting a electronsgenerated from said sample, comprising:at least two electron detectorsprovided between said electron source and said sample; a first targetplate located between said electron source and said two electrondetectors and having aperture for passing through said primary electronbeam; and at least two-staged deflecting electrode for deflectingelectrons is provided to each of said two electron detectors.
 12. Ascanning microscope according to claim 11, wherein said first conductivetarget plate is a flat panel.
 13. A scanning microscope according toclaim 11, further comprising a magnetic coil for generating a magneticfield in a direction crossing the electric field formed by said primaryelectron beam and said electrode.
 14. A scanning microscope according toclaim 13, wherein said magnetic field generated by said magnetic coilacts to suppress deflection of said primary electron beam by saidelectric field.
 15. A scanning microscope according to claim 9, furthercomprising a negative voltage applying power source for applying anegative electrical potential to said sample.
 16. A scanning microscopeaccording to claim 11, wherein a scan image is formed by using adetection signal from said two electron detectors or combining saiddetection signal of said two electron detectors.
 17. A scanningmicroscope according to claim 11, wherein the sole use a detectionsignal from one of said two electron detectors or the combined use ofsaid two electron detectors is automatically selected according to ascanning magnification or given observatory conditions.
 18. A scanningmicroscope according to claim 11, wherein said scanning deflector isserved through the effect of a combination of electrostatic deflectionand magnetic field deflection and is adjusted to apply desirousdeflection to said primary electron beam but no deflection to saidelectrons extracted from said sample.
 19. A scanning microscopeaccording to claim 11, further including means for applying a negativevoltage onto said sample.
 20. A scanning microscope according to claim11, wherein an acceleration tube is disposed in an electron beam passageof said objective lens, and said acceleration tube further includesmeans for providing voltage for accelerating said primary electron beam.21. A scanning microscope according to claim 11, further comprising asecond conductive target plate having an aperture for passing throughsaid primary electron beam provided between said two electron detectors.22. A scanning microscope according to claim 21, wherein said first andsecond conductive target plates are flat panels.
 23. A scanningmicroscope according to claim 21, further comprising a magnetic coil forforming a magnetic field in a direction crossing to an electric fieldformed by said said electrode.
 24. A scanning microscope according toclaim 23, wherein said magnetic field formed by said magnetic coil actsto suppress deflection of said primary electron beam caused by saidelectric field.
 25. A scanning microscope according to claim 21, whereinsaid scanning deflector is provided between said first and secondconductive target plate.
 26. A scanning microscope according to claim21, further comprising a negative voltage applying power source forapplying a negative potential to said sample.
 27. A scanning microscopeaccording to claim 21, wherein a scan image is formed by using adetection signal from one of said two electron detectors or combiningthe detection signal of said two secondary electron detectors.
 28. Ascanning microscope according to claim 21, wherein the use of adetection signal from only one of said two electron detectors or thecombined use of both of said two electron detectors is automaticallyselected according to a scanning magnification or given observatoryconditions.
 29. A scanning microscope according to claim 21, whereinsaid scanning deflector is served through the effect of a combination ofelectrostatic deflection and magnetic field deflection and is adjustedto apply desirous deflection to said primary electron beam but nodeflection to said electrons extracted from same sample.
 30. A scanningmicroscope according to claim 21, further including means for applying anegative voltage onto said sample.
 31. A scanning microscope accordingto claim 21, wherein an acceleration tube is disposed in an electronbeam passage of said objective lens, and said acceleration tube furtherincludes means for providing voltage for accelerating said primaryelectron beam.